And here I thought I could go all the way to the end without upgrading anything except movement speed... and then for some reason I got stuck on level 16 so I had to cave and upgrade all the way...
While I was playing I have no idea what I did, maybe I was too slow or something but when I went back to the hub there was a black shadow of a man standing on the left side. It scared the crap out of me and after I entered another room and went back to the hub there was nothing there. Also I like the support of Newton's laws of motion!!